1. Field of the Invention
The present invention relates to a support assembly for a deformable sealing element of a downhole tool, a downhole tool having a deformable sealing element, and to a method of supporting a deformable sealing element of a downhole tool. In particular, but not exclusively, the present invention relates to a support assembly for a deformable sealing element of a downhole tool such as a packer or a bridge plug.
2. Description of Related Art
As is well known in the oil and gas exploration and production industry, a wellbore is drilled from surface in order to gain access to subterranean hydrocarbon deposits (oil and gas). During the drilling and completion of a wellbore, it is frequently necessary to isolate and thus seal-off a portion of the wellbore. For example, the production tubing is typically located within and sealed relative to the casing/liner using a ‘packer’. Packers are also used in other downhole procedures, including intervention operations, where a remedial action is to be carried out downhole. Packers typically include a deformable sealing element which, when the packer is activated, are compressed axially, urging the sealing element radially outwardly into sealing abutment with an inner wall of the casing/liner.
Other types of downhole tools include similar deformable sealing elements. Typical such tools include ‘bridge plugs’ used to isolate part of a wellbore, and ‘straddles’ which may be required in circumstances where a tubing has corroded, leading to unwanted fluid ingress/egress. Where a tubing has corroded, a straddle including a pair of spaced deformable sealing elements is run downhole to straddle across the corroded section. A first such element is located uphole of the corroded section, and a second such sealing element downhole of the corroded section. In this fashion, when the straddle is activated, the sealing elements are both urged outwardly into sealing engagement with the inner wall of the tubing, to bridge across and isolate the corroded section, thereby preventing further unwanted fluid ingress/egress.
Typically, the deformable sealing elements of tools such as packers, bridge plugs, straddles and the like are of an elastomeric material which, when compressed axially, deform radially outwardly into abutment with the respective downhole tubing. If or when it is desired to remove the tools from the wellbore, the tool is actuated to release an applied compressive axial loading, moving the sealing element out of abutment with the tubing in question, so that the tool may be returned to surface.
However, loads applied to the sealing element when it is compressed and urged into abutment with the tubing can cause a permanent deformation of the sealing element, making it difficult to retract the element from abutment with the tubing, thereby hampering return of the tool to surface. If the element does not retract sufficiently the tool may stick as the element, when retracted, is sized to fit through any restrictions in the tubing.
Furthermore, the loads applied to the sealing elements can result in the sealing element extruding axially along the wellbore, reducing the sealing effect and potentially leading to seal failure.
In an effort to address these problems and deficiencies, it has become common practice to incorporate ‘garter’ springs into axial ends of the sealing elements. Garter springs typically comprise inner and outer coil springs wound in opposite directions which are moulded into annular elastomeric sections of a harder, less compressible material than a main portion of the sealing element, which are bonded to the main portion. In use, it has been found that, following deformation of the sealing element, the outer and inner coil springs tend to become interengaged, and elastomeric material tends to penetrate the coils. This results in the garter springs becoming permanently deformed such that, when a deformation load applied to the sealing element is removed, the garter springs do not completely retract to their undeformed positions. This ultimately leads to a permanent deformation of the sealing element and the problems highlighted above. Also, this permanent deformation requires complete replacement of the sealing element before the tool can be reused.
The present applicants have proposed an improved support assembly for a deformable sealing element in UK patent application no. 0713919.9, being incorporated herein by reference. In an embodiment, there is shown an arrangement with two thin expandable rings positioned either side of a sealing element. Under axially compressive loads, the rings are designed to expand along with the element until they touch the inner surface of the tubing. The ring's primary role is to give the elastomeric sealing element strength when trying to bridge a gap between the outer diameter of the bridge plug or packer and the inner diameter of the tubing or casing. The rings ability to expand is due to a series of relatively narrow slots cut into the rings by means of laser or wire eroding methods. This slot pattern allows the rings to expand, and if designed correctly will allow the rings to retract due to the elasticity of the ring material. However, in practice, though this arrangement worked well with regards to allowing the element to survive extrusion failure and pressure tests were performed with ease, some elastomeric extrusion into the slots occurred and this led to problems when recovering the sealing system through a restriction. When subjected to pressure differentials and elevated temperature, elastomeric material migrated into the slots on these rings (which are slightly wider when expanded) greatly reducing the rings ability to retract when loads were removed.
It is amongst the objects of at least one embodiment of the present invention to obviate or mitigate at least one of the foregoing disadvantages.